During manufacture of integrated circuit wafers, the wafer surface is sometimes treated by a chemical - mechanical planarization ("CMP") process. The treatment, also called chemical - mechanical polishing, is carried out by a so called CMP machine.
An example of a typical CMP machine is described in U.S. Pat. No. 5,702,292 to Brunelli et al. During polishing the CMP machine brings the wafer surface in contact with a rotating polishing pad under a biasing force.
Some CMP machines maintain the wafer in a wafer holder that faces up for loading the wafer on it. The wafer holder then faces down for polishing the wafer. During the reorientation the wafer is prevented from falling off by using a vacuum, as is taught in U.S. Pat. No. 5,095,661 to Gill et al.
Referring to FIGS. 1A and 1B, the wafer is preferably maintained in a recess of the wafer holder, as is also taught in U.S. Pat. No. 5,597,346 to Hempel, Jr. Specifically, a wafer holder 300 defines a recess 302 for receiving therein a wafer 304. The recess is bounded by a rim that decreases in height away from the recess.
During polishing, the biasing force enhances friction, which causes the wafer to shift laterally within the recess, in spite of the vacuum. Shifting is unavoidable, because the diameter of the recess must be larger than that of the wafer, so that the recess can receive the wafer in the first place. The diameter difference "d" determines the extent of shifting.
The lateral shifting results in uneven polishing, which limits how well the wafer can be polished. Uneven polishing results in local areas of over-polishing and under-polishing, that interfere with photolithographic etching processes for making integrated circuit structures. It also results in an uneven thickness of the planarization layer of the wafer. This does not permit a good functional die to be achieved from the wafer, and makes it difficult to maintain fine resolution tolerances in the wafer.